THE STRUCTURE OF XENIA HICKSONI. 201 



ino' the mesogloea in all parts of the tentacles and body of the polyps. 

 In the tentacles, however, these cells are few in number, but in the 

 body of the polyp, where the mesogloea is thicker, the cells are more 

 numerous. They are usually elongated or fusiform cells, having their 

 outer ends embedded in or connected with the ectoderm, and their 

 tapering inner ends passing into the endoderm (Fig. 17). In most 

 cases the connection between the two cell-layers is established by 

 means of a single cell, but in some cases two or more cells are placed 

 end to end to form the connecting cord. In most cases the larger 

 portion of the cell and its nucleus are situated in the ectoderm, and 

 this, together with the nature of the cell, which closely resembles an 

 ectoderm cell in appearance, points to the fact that these cells in the 

 mesogloea are derived from the ectoderm. Besides these moderately 

 large cells, the protoplasm of which is finely granular and often 

 contains a few small vacuoles, there are other cells of very much smaller 

 size which bear several or many processes. Some of the cells lie close 

 to the ectoderm, while others are near the endoderm. They are con- 

 nected together by their exceedingly slender processes, w T hich traverse 

 the mesogloea and unite with each other. These cells resemble, and 

 are probably homologous with, the nerve-cells and nerve-fibres of 

 Alcyonium and the Actiniae. They will be further described below (see 

 p. 209). The mesogloea of the mesenteries and its included cells have 

 already been described (see p. 197). 



2. Of the Stem (see PI. XX., Figs. 8 and 9). — On examining trans- 

 verse sections of the upper portion of the stem there is seen a slightly 

 denser ring of mesogloea (Mg. D.) around each of the coelentera. This 

 ring of denser mesogloea is itself moderately free from cells, being 

 crossed only at intervals by a cell or thin cord of cells, but it is bor- 

 dered by an almost complete cordon of cells, interrupted only for the 

 passage of endodermic canals. The canals of the stem are moderately 

 large and very numerous, being much more highly developed than 

 those of Alcyonium. The canals may be divided into the two systems 

 described below. 



The Superficial Canal System. — This system of canals is formed by 

 numerous endodermic canals (Sup. Can., Figs. 8 and 9) which are 

 situated in the stem about ■ 1 mm. beneath the ectoderm. This system 



